It all depends on the situation, the cop's grasp of the law (i.e: how bored they are that day), and how obviously educated you are about your own rights.

If you are not under arrest, there is no probable cause, and yet a cop is telling you he is calling a drug dog in... technically you can politely tell him you'll be leaving. I think a lot of the time people make the mistake of waiting around for the dog to get there. Depending on how the law is read that could be considered consent to search although I doubt your officer will take the time to explain that to you.

There is a lot of information about this in NORMLâ??s various "know your rights" pamphlets. Google!